An improved height control system for adjusting the elevation of a harvesting unit supportd for vertical movement on a cotton harvester. The height control system includes a signal receiving apparatus movable within a range of movement and arranged in combination with a lift mechanism for adjusting the elevation of the harvesting unit. A signal transmitting mechanism carried on the harvesting unit for ground engagement is connected to the signal receiving apparatus for causing the lift mechanism to effect harvesting unit elevation correlated with vertical movement of the signal transmitting mechanism relative to the harvesting unit. A linkage assembly interconnects the signal receiving apparatus and signal transmitting mechanism. The linkage assembly includes a lost motion mechanism for permitting relative movement between the signal transmitting mechanism and signal receiving apparatus following the signal receiving apparatus reaching on extreme limit of its travel thereby allowing for continued travel of the signal transmitting mechanism beyond a normal range of travel and thereby inhibiting damage to the component parts of the height control system.